1. Technical Field
This disclosure relates to antilock brake systems and, more specifically, to electrohydraulic antilock brake systems.
2. Description of the Related Art
Braking a vehicle in a controlled manner under adverse conditions such as rain, snow, or ice generally requires precise application of the brakes by the vehicle driver. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure, thereby causing the wheels to lockup and slip or skid on the road surface. Wheel lockup conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout.
In a continuing effort to improve the operational safety of vehicles, antilock braking systems have been developed. While such systems are adapted to control the braking of each braked wheel of a vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels. Generally, antilock brake systems are electrohydraulic and include a controller and sensors for monitoring the speed of the controlled wheels to determine the deceleration of the controlled wheels. Antilock brake systems also include one or more hydraulic circuits for applying pressure to the brakes of the controlled wheels. When the brakes of the vehicle are applied and the wheel deceleration of the monitored wheels exceeds a predetermined deceleration threshold, indicating that there is wheel slippage and the wheels are approaching a lockup condition, the controller functions to control the application of hydraulic pressure through a series of valves associated with the brakes to prevent a lockup of the controlled wheels. Typically, the controller will deactivate and activate the valves to cyclically release and reapply pressure to the brakes to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
Dual-circuit hydraulic antilock brake systems are widely used in vehicles which have two groups of wheels equipped with brakes, i.e., front and rear brakes. Dual-circuit hydraulic antilock brake systems include a pair of hydraulic brake circuits which are hydraulically independent of each other. One brake circuit is adapted to establish a braking pressure to be applied to the front wheel brake cylinders, while the other brake circuit is adapted to establish another independent braking pressure to be applied to the rear wheel brake cylinders. An example of such a dual-circuit hydraulic antilock brake system is disclosed in U.S. Pat. No. 4,824,183.
Because modem antilock brake systems are not purely hydraulic, but are electrohydraulic, in the event of an electrical failure a controller malfunction, an electrohydraulic antilock brake system will not operate properly. Because such an electrical failure will present a safety hazard, a reliable means for converting an electrohydraulic antilock braking system to a hydraulic braking system is needed in the event of an electrical failure or controller malfunction.